This invention relates to the preparation of inorganic xerogels.
Inorganic gels are well known and have long been used for various purposes, for example, for the adsorption of condensible vapors from gaseous carriers and for catalytic purposes either as a catalyst itself or as a component thereof or as a carrier. The most widely used of these inorganic gels in the dried condition is that of silica with or without the addition of other gelatinous materials such as alumina.
Inorganic gels in combination with various compounds, such as a chromium compound for example, have been found to be particularly useful as catalysts for carrying out reactions involving hydrocarbons, especially olefins such as ethylene, as disclosed in Dietz U.S. Pat. No. 3,887,494 (1975). It is known that the melt index and thus the molecular weight of an olefin polymer generally is responsive to the average pore diameter of the catalyst support. By increasing the average pore diameter of the catalyst support, therefore, there is achieved an increase in the melt index of the polymer prepared in the presence of a catalyst containing such support.
The manner in which a catalyst or support to be utilized in a catalytic composition is prepared can influence the average pore diameter of the support, and hence the melt index of the polymer prepared in the presence of the catalyst composition. Methods of preparing catalysts and inorganic hydrogels used in preparing catalyst compositions are well known in the art, such as those disclosed in Dietz U.S. Pat. No. 3,887,494 (1975) and Kimberlin et al U.S. Pat. No. 2,503,913 (1950).
In the preparation of an inorganic catalyst support (xerogel) from a hydrogel, e.g., a silica-containing hydrogel, an important step is the removal of water from the hydrogel or the drying of the hydrogel to produce the xerogel. The step is important in that the manner in which the water is removed has a great bearing upon the size of the pores of the catalyst support (xerogel). One method of drying hydrogel is to heat the hydrogel at a high temperature in order to evaporate the water. A problem, however, is that the dried hydrogel (xerogel) has relatively small pores.
The prior art has somewhat overcome this problem by removing the water from the hydrogel through repeated washings with an organic compound or by azeotropic distillation. The organic compounds utilized are usually aliphatic monohydric alcohols or ketones, as disclosed in Kearby U.S. Pat. No. 2,429,319 (1947), Kimberlin et al U.S. Pat. No. 2,503,913 (1950) and Wetzel et al U.S. Pat. No. 2,978,298 (1961). Use of these organic compounds requires many time-consuming steps to recover a dried gel. Azeotropic distillation involves not only additional processing but also additional expense.
An object of the present invention, therefore, is to provide a simple process to produce an improved inorganic xerogel.
Other objects, aspects and the several advantages of this invention will be apparent to those skilled in the art upon a study of this disclosure and the appended claims.